Why would you use two different calculations for the US and China?
if Chinese average growth rate is 5.2% (by simply adding both years 2.3+8.1 then dividing by two) then US average growth rate is 1.05% (-3.5+5.6 then divide by two).
Chinese = 5.2% average annual growth rate
US = 1.05% average annual growth rate
China's average growth rate was 4.95x as much (+395% more) over that two year period.
Of course this doesn't actually tell you how much each country grew, just the average rate of growth (and not even taking into account compounding).
As @manqiangrexue states, the more appropriate way to calculate is the difference between starting point and ending point (you actually used this for the US in your comment, but then you calculated only the average annual growth rate for China and used that to compare to the total growth of the US over the two year period):
US is 1.9% larger over the two year period
China is 10.6% larger over the two year period
China grew 5.58x as fast (458% more) over the two year period of 2020 and 2021 as compared to the US.
What's amazing is China is so big now (almost 80% of the US in annual nominal GDP, including HK/MO) that growth differentials like that create an immense difference in economic output added. If China grew 5.58x as much in the next two year period again for example, it would add 4.46x as much in gross value.
So if the US added $400 billion to its GDP over a two year period, China would add $1.784 trillion.
That's correct, I made a mistake in the calculation.
If you take into account compound, US growth was less than 1% average.
US nominal GDP growth this year is going to be huge because of 7% inflation. That's going to add many percentage points all by itself. Another example of why nominal GDP is ridiculous.